1. Field of the Invention
The present invention relates to an EL (electroluminescence) panel, an EL phosphor thin film used therefor, and a method of making the same.
2. Related Background Art
In recent years, research has actively been under way concerning thin-film EL devices as a small-size, or large-size, lightweight flat display panel. FIG. 5 is a perspective view showing an example of conventional EL displays. This EL panel 100, which is a monochromatic thin-film EL display comprising a phosphor thin film made of yellowish-orange-emitting manganese-doped zinc sulfide (ZnS:Mn) disposed between two thin-film insulating layers and having a so-called double insulating structure, has already been in actual use.
Specifically, a lower electrode 13a having a predetermined pattern is formed on a substrate 12 made of glass, whereas a dielectric thin film is formed as a lower insulating layer 14a on the lower electrode 13a. A light-emitting layer 15 made of a phosphor thin film and an upper insulating layer 14b are successively formed on the lower insulating layer 14a. On the upper insulating layer 14b, an upper electrode 13b is formed with a predetermined pattern so as to construct a matrix electrode in cooperation with the lower electrode 13a. For improving luminance, the light-emitting layer 15 is usually annealed at a temperature not higher than the strain point after the phosphor thin film is formed.
A structure using a substrate 12 made of ceramics and employing a thick-film dielectric layer as a lower insulating layer 14a has recently been proposed as well. Also proposed is a device structure in which a BaTiO3 thin sheet, formed with an electrode on one side, exhibiting a high dielectric constant is used as both lower insulating layer 14a and substrate 12. Since such a structure uses ceramics such as alumina or BaTiO3 as the substrate, the light-emitting layer 15 made of a phosphor thin film can be annealed at a higher temperature, whereby higher luminance can be attained.
Further, since a dielectric layer constituted by a thick film or thin sheet is used as the lower insulating layer 14a, it is stronger against dielectric breakdown than are those using a thin film for the lower insulating layer 14a, and thus is advantageous in that a highly reliable device is obtained.
The double insulating structure is not always necessary, and a thick-film or thin-sheet dielectric layer may be provided alone as an insulating layer.
Meanwhile, displays currently in use for personal computers, TVs, and other displaying purposes are required to show colors. Though thin-film EL displays using a zinc sulfide phosphor thin film are excellent in reliability and environmental resistance, they have not yet been considered suitable for color displays, since characteristics of EL phosphors emitting three primary colors of red, green, and blue are not sufficient.
Here, the following materials have been listed as candidates for materials for red, green, and blue, and their studies are underway. Examples of blue-emitting phosphors include SrS:Ce using SrS as a matrix material and Ce as a luminescent center, SrGa2S4:Ce, and ZnS:Tm. Examples of red-emitting phosphors include ZnS:Sm and CaS:Eu. Examples of green-emitting phosphors include ZnS:Tb and CaS:Ce.
These phosphor thin films emitting three primary colors of red, green, and blue have not yet been sufficient in terms of light-emitting luminance, efficiency, and color purity, so that color EL panels using them have still failed to come into actual use. Among them, red emission with a relatively favorable color purity has been attained by using CaS:Eu, and various improvements thereon have been proposed as disclosed in Japanese Patent Application Laid-Open Nos. HEI 1-206594 and HEI 2-148688, for example. However, they have not yet been sufficient as red-emitting materials for a full-color display in terms of light-emitting characteristics such as luminance and light-emitting efficiency.
On the other hand, CaS:Eu has a relatively long response time of several seconds to several tens of seconds as disclosed in Japanese Patent Application Laid-Open No. HEI 2-51891 and ITEJ Technical Report, Vol. 16., No. 76, pp. 7-11, thus failing to come into actual use as it is for displaying moving images requiring real-time responses to driving signals.
Under such circumstances, a ZnS:Mn phosphor thin film exhibiting high luminance and efficiency is used in general for red emission, and the red wavelength band is taken out from the EL spectrum of the phosphor thin film through a color filter, whereby red color necessary for a full-color panel is obtained. Using a color filter not only complicates the manufacturing process, but also yields an essential and important problem in that luminance decreases. When taking out red color by using a color filter, luminance tends to decrease by 10% to 20%, for example, thereby becoming insufficient for actual use.
As mentioned above, sulfide composition systems expanded from zinc sulfide (ZnS) to double and triple sulfides have widely been used so far as phosphors for EL devices. In general, however, sulfide type materials have the drawback of being susceptible to water and moisture. For example, Al2S4 reacts with water in the air, thereby changing into Al2O3 while generating H2S. Therefore, when employing a phosphor made of a sulfide type material in an EL device, means for blocking water and protecting the device must be provided from the viewpoint of phosphor life. This may complicate the device structure.
Under these circumstances, the research and development for high-luminance EL devices using chemically stable oxide type phosphors in light-emitting layers has recently been gaining momentum. EL materials with various emission colors have been proposed, for example, in Display and Imaging, Vol. 8 suppl., pp. 83-93, Japanese National-Phase International Patent Application Laid-Open No. HEI 11-508628 (International Publication WO97/02721 pamphlet), and Japanese National-Phase International Patent Application Laid-Open No. 2001-512406 (International Publication WO98/33359 pamphlet).